1. Field of the Invention
The present invention relates to securing devices adapted to limit the tension in a rope or the like such that the tension remains below a predetermined maximum value.
2. Discussion of Prior Art
The term rope or the like which is used throughout the specification is intended to include metal and fibre stranded rope, unstranded flexible rope substitutes and chains.
When ropes or the like are used to moor a vessel to a land-based ground anchor, frequent adjustment of the moorings is often necessary to take account of rising and falling tides. If the vessel is securely moored at high tide, then as the tide falls the situation may be reached in which the moorings effectively support the vessel resulting in possible failure of either the rope or the like or the anchor. If the vessel is a section of a floating bridge, a similar situation may occur when a large load passes over the bridge. The consequent lowering of the vessel in the water may result in failure of the mooring or anchor. Both of the above problems are increased if the moorings have a significant vertical component. This situation frequently arises when there is a requirement to bridge a river having steep banks with a floating bridge.
The complete release of a rope or the like in a situation in which overload is imminent is clearly undesirable and a limited release of the rope or the like in order to reduce its tension to a safe level is clearly preferable.
The problems of securing a rope or the like in such a manner that its tension is limited to a predetermined maximum value have been addressed in a number of ways in the past.
One method of partially mitigating the above problems is to install a spring at some point in the mooring. There are a number of disadvantages associated with this solution: (a) such springs are very costly; (b) the maximum possible extension of the mooring is severely limited by the necessity for the spring to be easily handled; and (c) the tension maintained in the mooring will increase as the energy stored in the spring is increased.
A hydraulic chain tensioning device is disclosed in patent specification GB 1065641. In this device a tensioned chain is connected to a piston which urges fluid from a cylinder past a pressure relief valve. Such a device can maintain a constant tension in the chain but still only provides an extension limited by the stroke of the piston and cylinder device.
In order to overcome the limited extension problem of the GB 1065641 device GB 2097040 proposes a chain tensioning device for hauling mining machinery. The device incorporates a hydraulic motor and a hydraulic pump connected by circuitry incorporating a chain tension sensor. The device itself does not limit the length by which the chain can be extended however the complexity and consequential size means that the device is not easily portable.
An anchoring device is disclosed in Russian patent 1065286 in which a rope is gripped between two rollers. Each roller has at its lower end an eccentric mounting for opposite ends of a hydraulic piston and cylinder. Rotation of the rollers causes reciprocatory movement in the piston and cylinder causing fluid to flow from one end of the cylinder to the other through a pressure relief valve. A major disadvantage of this device is that the rope being anchored needs to be of a precise size to be gripped successfully. If the rollers or rope become worn inadequate gripping of the rope will take place, and a rope slightly larger than the gap between the rollers will not be insertable between the rollers.